Rotary impact motor

ABSTRACT

In a bidirectionally operable impact motor for power driven fastener setting tools an impact dog is journaled in a hammer body pivotally about an axis spaced from but parallel with the axis of rotation of the hammer body for taking impact and release positions with respect to opposed impact surfaces on a rotatable anvil coaxial with the hammer body, one impact surface for each direction of rotation of the impact motor. A driving member rotates the hammer body via the impact dog and during rotation constantly biases the latter to the release position. A cam body is pivotally supported coaxially on the anvil between two end positions. When the direction of rotation of the impact motor is changed, the cam body is pivoted from one of the end positions to the opposite one for purposes of camming engagement with the impact dog for pivoting it to a correct impact position against the one of the impact surfaces that becomes active during the chosen direction of rotation.

1 n i i lnitnd States intent [is] 3,nl8,784

finhoens 1 Mar. M, i972 i541 RUTAMW IMPACT MUTUR Primary Examiner-James A. Leppink {72] Inventor. lKnut Christian Schoeps, hlacka, Sweden Atwmey Bauer and Goodman 1'13] Assignee: Atlas Copoo Aktieholag, Nacka, Sweden 221 Filed: Sept. 18, 1970 [57] ABSTRACT U In a bidirectionally operable impact motor for power driven U1] Appl' 73552 fastener setting tools an impact'dog is joumaled in a hammer body pivotally about an axis spaced from but parallel with the [30] Foreign Application Priority Data axis of rotation of the hammer body for taking impact and release positions with respect to opposed impact surfaces on a Sept. 26, 1969 Sweden ..l3304/69 rotamble anvil coaxial with the hammer body, one impact Sup face for each direction of rotation of the impact motor. A driv Efnil ..173/9%g,58ill//2d3 ing member rotates the hammer body via the impact dog and {'58} a 81/52 3, during rotation constantly biases the latter to the release position. A cam body is pivotally supported coaxially on the anvil between two end positions. When the direction of rotation of References cued the impact motor is changed, the cam body is pivoted from UNlTED STATES PATENTS one of the end positions to the opposite one for purposes of camming engagement with the impact dog for pivoting it to a 2,940,566 6/1960 COnOVel'i 173/93 5 correct impact position against the one of the impact surfaces 3,179,219 4/1965 Karden 173/93 5 that becomes active during the chosen direction of rotation. 3,321,043 5/1967 Vaughn ...l73/93.5 3,557,884 6/1971 Allen ..l73/93.5 110 Clanns, lll Drawing Figures PAIENTEUMAR 14 I972 3, 648 J84 am 1 UF 2 mg: Fig- 4 INVEN'I'OR.

PAIENTEDMARMIQYZ Y 3.648.784

SHEET 2 BF 2 INVEN'I'UR.

ROTARY IMPACT MOTOR This invention relates to a bidirectionally rotatable rotary impact motor with impacting action in either rotational direction thereof. More specifically the invention relates to rotary impact motors of the type with a hammer body coaxially rotatably supported with respect to the anvil, and an impact dog provided on the hammer body and pivotally arranged on the hammer body about an axis parallel with the rotational axis of the anvil for taking respectively impact and release positions with respect to the impact surfaces of the anvil under the action of cooperating cam means on the anvil, the impact dog, and on a driving member for rotating the hammer body via the impact dog.

In such rotary impact motors the driving member usually constantly strives to pivot the impact dog back in the release direction so that the end of the impact dog trailing in the rotational direction during the relation rotation between the impact dog and anvil is forced to follow the periphery of the anvil. Movement of the impact dog to an impact position is controlled by cam action and for purposes of promoting such action the impact surfaces of the anvil are placed peripherally as near as possible to one another, i.e., on a mutual distance cor-' responding to the width of the impact dog and the cam means of the anvil are correspondingly divided into two peripherally spaced opposed cam crests, one cam crest for each impact surface and direction of rotation. When the impact dog has to pass over the second cam crest after the pivotal movement to an impact position over the first one, the impact dog must be pivoted a second time. In order to attain a sufficiently large angle of acceleration prior to impact, it is desired to strike only once for each revolution, and therefore this second pivotal movement is allowed to take place without impacting. This parasitic oscillation uses up energy and demands special radial depressions to be formed in the anvil in order to provide clearance for the pivotal oscillation of the impact dog, such measures increasing the cost of the anvil and weakening it.

It is an object of the invention to provide a rotary impact motor of the above defined type which is particularly suitable to deliver only a single impact per revolution free from parasitic oscillations and consequent constructive weakening and loss of driving power. A further object of the invention is to provide a simple and stable journaling of the hammer body and the other details of the rotary impact motor and to rely only on a minimum of details involved.

SUMMARY OF THE INVENTION For the above and other purposes there is according to the invention provided a bidirectionally rotatable rotary impact motor with impacting action in either rotational direction thereof comprising a housing, an anvil rotatably journaled in said housing, a pair of peripherally opposed impact surfaces on said anvil each corresponding to one of said rotational directions, a hammer body rotatably carried in said housing coaxially with respect to the axis of rotation of said anvil, an impact dog supported on said hammer body for pivotal movement about an axis spaced from but parallel with said axis of rotation for taking respectively impact or release positions relative to said impact surfaces, cam means on said impact dog, a rotatable driving member in said housing in camming engagement with said impact dog for rotating said hammer body in either direction and pivoting said impact dog to said release position, and a cam body cooperating with said cam means and supported movably in a peripheral direction on said anvil between two end positions thereon for pivoting in each of them said impact dog to an impact position against the one of said impact surfaces corresponding to the chosen rotational direction ofsaid impact motor.

The above and other purposes of the invention will become obvious from the following description and from the accompanying drawings in which two embodiments of the invention are illustrated by way of example. It should be understood that these embodiments are only illustrative of the invention and that various modifications thereof may be made within the scope of the claims following hereinafter.

In the drawings,

FIG. II is a longitudinal section through a rotary impact motor according to the invention with the details included therein shown in impact position.

FIG. 2 is a section on the line 2-2 in FIG. I restricted to the details in the hammer body of the impact motor.

FIG. 3 is a corresponding section on the line 3--3 in FIG. I.

FIG. 4 is a perspective view of an impact dog in FIG. I together with the cam means of the anvil cooperating therewith.

FIG. 5 is a section corresponding to FIG. 2 but of the impact motor in release position immediately after impacting.

FIG. 6 is a section corresponding to FIG. 3 but with the appurtenant details in release position.

FIG. 7 and FIG. 8 are sequential] sections showing the details in FIG. 5 during acceleration up to a position immediately prior to the impact position illustrated in FIG. 2.

FIG. 9 shows the details in FIG. 2 in impact position upon reversal of the rotating and impacting direction of the impact motor.

FIG. 10 shows a fragmentary rear end view of an anvil with a modified impact dog and modified cam means therefor.

FIG. lll finally shows a perspective view corresponding to FIG. 4 but ofthe cam means and of the impact dog in FIG. 10.

In the figures the impact clutch is shown as fitted in a hand sustained impact wrench 15 which includes a housing I6 and for example a reversible pneumatic motor, not shown in the figures, the motor causing rotationin the desired rotational direction of a drive shaft 17 journaled in the housing. The impact motor or impact clutch mechanism proper is interposed between the housing 16 and a front piece 18 which within a journal bore 19 rotatably supports a forwardly projecting anvil 20. The forward end of the anvil 20 carries a driving end 21 intended for removably carrying a tool thereon, not shown, usually a socket wrench.

The drive shaft 17 is forwardly provided with splines and spline grooves 22 and receives in an inner bore rotatably a centering pivot 23 which is fixed centrally in the rear end portion 24 at the rear end face of the anvil 20. The end portion 24 is circularly cylindrical and provided with an axial partly cylindrical recess 25, the radially outwardly projecting outer portions of which form a pair of opposed impact surfaces 26, 27, FIG. 7.

By means of a journal bore 29 a hammer body 28 is coaxially rotatably journaled on the end portion 24 of the anvil. Into the journal bore 29 there opens a partly cylindrical journaling recess 30 which slidably receives an impact dog 32 the latter being arched arcuately in cross section. The impact dog 32 has a convex, partly cylindrical back portion 33 which in its entire length is slidably supported by the cylindrical surface of the journaling recess 30 for pivotal movement about an axis parallel with the rotational axis of the hammer body 28 and of the anvil 20. The impact dog 32 has a recess 34 directed towards the end portion 24 which in a position of symmetry of the impact dog 32 within the hammer body 28, FIGS. 5, 7, allows the impact dog 32 to move freely with respect to the end portion 24 so that the hammer body 28 can rotate relative to the anvil 20 about the end portion 24, the latter thus being circumscribed by the path of movement of the impact dog 32. The impact dog 32 carries at the rear end thereof a single lobed cam body 35 being formed symmetrical with respect to the central longitudinal plane thereof and in one piece therewith and having a radially inwardly directed crest. Rearwardly the journal bore 29 of the hammer body 23 is transformed into an enlarged cylindrical journal portion 36.

On the splines and spline grooves 22 of the drive shaft 17 fits a driving element 38 by cooperating splines and spline grooves 37, the driving element slidably cooperating by a front cam flange 39 with the cylindrical journaling portion 36 of the hammer body 28. The cam flange 39 is provided with cut-out cam member consisting of opposite radially disposed cam surfaces 40,41 at opposite sides of the cam body 35 of the impact dog 32 and directed for cooperation with the radially outwardly disposed base portion thereof. Against an axial shoulder 42 within the driving element 38 is applied an intermediate washer 43 rotatably fitted onto the centering pivot 23. A single lobed anvil cam body 44 on the anvil 20 is pivotally journaled on said same centering pivot 23 surrounded by the cam flange 39 radially spaced therefrom and the cam body 44 is countered by said washer and inserted between the face of end portion 24 and the intermediate washer 43.

For receiving the cam body 44 of he anvil 20 the rear end or end portion 24 of the anvil 20 at the face thereof is provided with an axial preferably cylindrical recess 45 into which recess is inserted the base portion of the cam body 44 which has a lug 46 thereon. The base portion and the lug 46 are carried pivotally in the recess 45 between two end positions symmetrical to one another, respectively FIG. 2 and FIG. 9, in which end position the cam body 44 is arrested by an abutment in the form of a pin 47 cooperating with the lug 46. The pin 47 is press-fitted into a bore at the periphery of the recess 45 to such a depth that it can be freely passed by a cam crest 48 on the cam lobe of cam body 44 during pivotal movement of the cam body 44. The cam crest 48 lies axially outside of the recess 45 and projects radially into the path of movement of the impact dog cam body 35 for cooperation with the crest of the latter. During relative rotation between the hammer body 28 and the anvil 20, the anvil cam body 44 is circumscribed by the path of movement of the cam means on the driving element 38, that is to say of the cam surfaces 40,41 and is likewise circumscribed by the path of movement of the impact dog cam body 35. Thus it is seen that all the cooperating cam means 40, 41,44 and 35 are disposed in a common plane of rotation behind the end portion 24 of the anvil 20.

Let it be supposed that the drive shaft 17 in operation of the impact Wrench rotates the impact motor clockwise, when the details thereof are viewed in FIG. I in the direction of the arrows 22, and that the drive shaft 17 through the intermediary of the drive connection consisting of the splines and spline grooves 22, 37 and via the impact motor rotates the driving end 21 of the anvil, which driving end transmits rotation to a socket wrench and screw, not shown. As long as the screw rotates easily, the parts of the rotary impact motor by reason of friction will stay in the prevailing position, for example in the impact position shown in FIG. 2, in which the back portion 33 of the impact dog 32 is in engagement with the impact surface 26 of the anvil. The rotation of the driving element 38 is transmitted at the cam surface 40 to the cam body 35 of the impact dog 32 and thus the hammer body 28 is rotated while the impact dog rotates the anvil 20, the socket wrench, and the screw by engagement with the impact surface 26. Rotation goes on continuously until the screw has been screwed down and the resistance to rotation is increased.

At sufficiently large rotational resistance the anvil stops while the drive shaft 17 forces the driving element 38 to go on rotating relative to the anvil 20 and relative to the impact dog 32 temporarily arrested by the impact surface 26 of the anvil. The movement of the driving element 38 causes the cam surface 40, FIG. 3, to turn the impact dog 32 to the release position, FIGS. 5, 6. The back portion 33 of the impact dog 32 at such instant leaves the engagement with the impact surface 26 and the recess 34 in the impact dog 32 falls flush with or slightly outside of the cylindrical periphery of the end portion 24 with such a clearance that the cam flange 39 of the driving element 38 by the cam surface 40 can rotate the impact dog 32 and the hammer body 28 relative to the anvil 20 under angular acceleration past the position in FIG. 7 to the position in FIG. 8. Arriving to the latter the cam body 35 of the impact dog 32 hits the crest 48 of the anvil cam body 44 which causes an instant turning of the impact dog 32 to the impact position for meeting the impact surface 26 of the anvil 20, FIG. 2. Meanwhile theanvil cam body 44 is kept immovable relative to theend portion 24 by the abutment pin 47. In FIG. 2 the impact dog 32 by its back portion 32 delivers an impact under full velocity against the impact surface 26 and transmits the kinetic energy of the hammer body 28 and of the impact dog 32 to the end portion 24 of the anvil 20 in the form of a rotational impact. In sequence to the impact the impact dog 32 recoils back and at such instant can hit the crest 48 of the anvil cam body 44 by a blow directed in the opposite counterclockwise direction. In that direction, however, the cam body 44 is unrestricted to pivot about the pivot 23 away from the pin 47 and accordingly there cannot occur any interwedging or locking tendencies. Upon recoil the impact dog 32 is released by the cam surface 40 of the drive element 38 and the next acceleration cycle of the hammer body 28 is started in the above described manner and repeated blow after blow until the desired torque has been reached in the screw.

When the rotational direction is reversed for rotation in counter-clockwise direction, the details of the impact motor will occupy an impact position according to FIG. 9. At reversing, the cam body 35 of the impact dog 32 brings along the cam body 44 of the anvil at the cam crest 48 in a counterclockwise direction to the end position in FIG. 9 symmetrical with respect to the end position in FIG. 5 at which instant the lug 46 of the cam body 44 abuts against the opposite side of the abutment pin 47 as compared to FIG. 5. Thanks to the symmetrical design of the parts involved, impacting thereupon is performed in analogy with the impact cycle during clockwise rotation with the exception that the impact dog 32 by its back portion 33 now ponds against the impact surface 27 simultaneously with the driving element 38 actuating the impact dog cam body 35 from the opposite direction by the cam surface 41.

' In the embodiment depicted in FIGS. 10, 11 the rear end of the anvil presents a substantially triangular recess 49 rounded at the comers and with a flat side surface 50 which is perpendicular to the plane through the axes of rotation of the anvil 20 and the impact dog 32 at the instant of impact. The anvil cam body 44 has an angularly shaped lug 51 with flanks 52, 53. The lug 51 is countered arrestingly by the side surface 50 and applies itself thereagainst either by one flank 52 thereof or by the other flank 53 and thereby defines the end position of the cam body 44 respectively during rotation in a clockwise, FIG. 10, or in a counter-clockwise direction, in the latter case, as is obvious, in a position of symmetry with respect to FIG. 10, not shown. The cooperating impact dog 32, FIG. 11, is reinforced at the cam body 35 by a fully cylindrical reinforcing portion 54 between the cam body 35 and the recess 34 of the impact dog 32.

I claim:

1. A bidirectionally rotatable rotary impact motor with impacting action in either rotational direction thereof comprising a housing, an anvil rotatably journaled in said housing, a pair of peripherally opposed impact surfaces on said anvil each corresponding to one of said rotational directions, a hammer body rotatably carried in said housing coaxially with respect to the axis of rotation of said anvil, an impact dog supported on said hammer body for pivotal movement about an axis spaced from but parallel with said axis of rotation for taking respectively impact or release positions relative to said impact surfaces, cam means on said impact dog, a rotatable driv ing member in said housing in camming engagement with said impact dog for rotating said hammer body in either direction and pivoting said impact dog to said release position, and a cam body cooperating with said cam means and supported movably in a peripheral direction on said anvil between two end positions thereon for pivoting in each of them said impact dog to impact an position against the one of said impact surfaces corresponding to the chosen rotational direction of said impact motor.

2. A rotary impact motor according to claim 1 in which said cam body and cam means both are cooperating single lobed cams, said driving member having opposed cam surfaces engageable with opposite sides of said impact dog cam.

3. A rotary impact motor according to claim 1 in which said cam body of said anvil is supported at the rear end of said anvil pivotally about a pivot thereon and coaxial therewith.

4. A rotary impact motor according to claim 3 in which there is provided an axial recess around said pivot in said anvil, a base portion on said cam body extending into said axial recess, a lug on said base portion, and abutment means on said anvil in said recess and cooperating with said lug for defining said end positions of said cam body.

5. A rotary impact motor according to claim 2 in which said impact dog cam is symmetrical with respect to a central plane of symmetry for the impact dog, the end positions of said anvil cam being symmetrical with respect to a central plane of symmetry for said impact surfaces on said anvil.

6. A rotary impact motor according to claim 5 in which said impact surfaces of said anvil are provided by an axial recess in a circularly cylindrical rear end portion of said anvil about which the impact dog has its path of movement, said hammer body by means ofa central axial joumaling bore therein being rotatably supported on said end portion.

7. A bidirectionally rotatable rotary impact motor with impacting action in either rotational direction thereof comprising a housing, an anvil rotatably journaled in said housing, a pair of peripherally opposed impact surfaces on said anvil each corresponding to one of said rotational directions, a hammer body rotatably carried in said housing coaxially with respect to the axis of rotation of said anvil, an impact dog supported on said hammer body for pivotal movement about an axis spaced from but parallel with said axis of rotation for taking respectively impact or release positions relative to said impact surfaces, a single lobed cam on said impact dog, a rotatable driving member in said housing, opposed cam surfaces on said driving member engageable with opposite sides of said impact dog cam for rotating said hammer body in either direction and pivoting said impact dog to said release position, a cam body cooperating with said impact dog cam and supported movably in a peripheral direction on said anvil between two end positions thereon for pivoting in each of them said impact dog to an impact position against the one of said impact surfaces corresponding to the chosen rotational direction of said impact motor, and said impact dog cam, said cam surfaces, and said anvil cam body being substantially coplanar with said cam surfaces circumscribing by their path of movement said anvil cam during relative rotation between said hammer body and anvil.

8. A rotary impact motor according to claim 7 in which said cam surfaces are provided on a ring shaped cam flange on said driving member, said cam flange surrounding said anvil cam body and rotatably fitting in a cylindrical joumaling portion in and rearwardly of said hammer body.

9. A rotary impact motor according to claim 8 in which said impact surfaces of said anvil are provided by an axial recess in a circularly cylindrical rear end portion of said anvil about which the impact dog has its path of movement, said hammer body by means ofa central axial joumaling bore therein being rotatably supported on said end portion, and said anvil cam body being supported at the rear face of said end portion about a pivot coaxial with said anvil.

It). A bidirectionally rotatable rotary impact motor with impacting action in either rotational direction thereof comprising a housing, an anvil rotatably journaled in said housing, a pair of peripherally opposed semi cylindrical impact surfaces on said anvil each corresponding to one of said rotational directions, a hammer body rotatably carried in said housing coaxially with respect to and around said anvil, a semi cylindrical impact dog supported on said hammer body for pivotal movement about an axis spaced from but parallel with said axis of rotation for taking respectively impact or release positions relative to said impact surfaces, a cam on said impact dog, a rotatable driving member in said housing rearwardly of said hammer body, opposed cam surfaces on said driving member engageable with opposite sides of said impact dog cam for rotating said hammer body in either direction and biasing the trailing end of said impact dog towards said anvil and said release position of said impact dog, and a cam body cooperating with said impact do cam and supported movably in a peripheral direction on sat anvil between two end positions thereon for pivoting in each of them the leading end of said impact dog towards said anvil and said impact position of said impact dog against the one of said impact surfaces corresponding to the chosen rotational direction of said impact motor.

I UNITED STATES PATENT OFFICE CERTIFICATE OI CORRECTION Patent No 3,6 +8,784 Dated March 14, 1972 Inv'mor(s) Knut Christian Schoeps It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 66 delete "impact an" and insert an impact" Signed and sealed this 15th day .of' August 1972.

(SEAL) v Attest:

ROBERT GOTTSCHALK EDWARD M. FLETCHER, JR.

Cmpmis'sioner of Patents Attesting Officer- USCOMM-DC cone-P69 FORM POIOSO (10-69) 

1. A bidirectionally rotatable rotary impact motor with impacting action in either rotational direction thereof comprising a housing, an anvil rotatably journalled in said housing, a pair of peripherally opposed impact surfaces on said anvil each corresponding to one of said rotational directions, a hammer body rotatably carried in said housing coaxially with respect to the axis of rotation of said anvil, an impact dog supported on said hammer body for pivotal movement about an axis spaced from but parallel with said axis of rotation for taking respectively impact or release positions relative to said impact surfaces, cam means on said impact dog, a rotatable driving member in said housing in camming engagement with said impact dog for rotating said hammer body in either direction and pivoting said impact dog to said release position, and a cam body cooperating with said cam means and supported movably in a peripheral direction on said anvil between two end positions thereon for pivoting in each of them said impact dog to impact an position against the one of said impact surfaces corresponding to the chosen rotational direction of said impact motor.
 2. A rotary impact motor according to claim 1 in which said cam body and cam means both are cooperating single lobed cams, said driving member having opposed cam surfaces engageable with opposite sides of said impact dog cam.
 3. A rotary impact motor according to claim 1 in which said cam body of said anvil is supported at the rear end of said anvil pivotally about a pivot thereon and coaxial therewith.
 4. A rotary impact motor according to claim 3 in which there is provided an axial recess around said pivot in said anvil, a base portion on said cam body extending into said axial recess, a lug on said base portion, and abutment means on said anvil in said recess and cooperating with said lug for defining said end positions of said cam body.
 5. A rotary impact motor according to claim 2 in which said impact dog cam is symmetrical with respect to a central plane of symmetry for the impact dog, the end positions of said anvil cam being symmetrical with respect to a central plane of symmetry for said impact surfaces on said anvil.
 6. A rotary impact motor according to claim 5 in which said impact surfaces of said anvil are provided by an axial recess in a circularly cylindrical rear end portion of said anvil about which the impact dog has its path of movement, said hammer body by means of a central axial journalling bore therein being rotatably supported on said end portion.
 7. A bidirectionally rotatable rotary impact motor with impacting action in either rotational direction thereof comprising a housing, an anvil rotatably journalled in said housing, a pair of peripherally opposed impact surfaces on said anvil each corresponding to one of said rotational directions, a hammer body rotatably carried in said housing coaxially with respect to the axis of rotation of said anvil, an impact dog supported on said hammer body for pivotal movement about an axis spaced from but parallel with said axis of rotation for taking respectively impact or relEase positions relative to said impact surfaces, a single lobed cam on said impact dog, a rotatable driving member in said housing, opposed cam surfaces on said driving member engageable with opposite sides of said impact dog cam for rotating said hammer body in either direction and pivoting said impact dog to said release position, a cam body cooperating with said impact dog cam and supported movably in a peripheral direction on said anvil between two end positions thereon for pivoting in each of them said impact dog to an impact position against the one of said impact surfaces corresponding to the chosen rotational direction of said impact motor, and said impact dog cam, said cam surfaces, and said anvil cam body being substantially coplanar with said cam surfaces circumscribing by their path of movement said anvil cam during relative rotation between said hammer body and anvil.
 8. A rotary impact motor according to claim 7 in which said cam surfaces are provided on a ring shaped cam flange on said driving member, said cam flange surrounding said anvil cam body and rotatably fitting in a cylindrical journalling portion in and rearwardly of said hammer body.
 9. A rotary impact motor according to claim 8 in which said impact surfaces of said anvil are provided by an axial recess in a circularly cylindrical rear end portion of said anvil about which the impact dog has its path of movement, said hammer body by means of a central axial journalling bore therein being rotatably supported on said end portion, and said anvil cam body being supported at the rear face of said end portion about a pivot coaxial with said anvil.
 10. A bidirectionally rotatable rotary impact motor with impacting action in either rotational direction thereof comprising a housing, an anvil rotatably journalled in said housing, a pair of peripherally opposed semi cylindrical impact surfaces on said anvil each corresponding to one of said rotational directions, a hammer body rotatably carried in said housing coaxially with respect to and around said anvil, a semi cylindrical impact dog supported on said hammer body for pivotal movement about an axis spaced from but parallel with said axis of rotation for taking respectively impact or release positions relative to said impact surfaces, a cam on said impact dog, a rotatable driving member in said housing rearwardly of said hammer body,opposed cam surfaces on said driving member engageable with opposite sides of said impact dog cam for rotating said hammer body in either direction and biasing the trailing end of said impact dog towards said anvil and said release position of said impact dog, and a cam body cooperating with said impact dog cam and supported movably in a peripheral direction on said anvil between two end positions thereon for pivoting in each of them the leading end of said impact dog towards said anvil and said impact position of said impact dog against the one of said impact surfaces corresponding to the chosen rotational direction of said impact motor. 